This invention concerns improvements in or relating to optical apparatus and relates more particularly to optical apparatus for biocular viewing. More particularly it relates to optical apparatus for biocular viewing by which an observer looking into the apparatus with his eyes, located at spaced eye positions, can see with both eyes a magnified image of visual information displayed on an image surface.
Optical apparatus for biocular viewing is used, for example, in night vision equipment by which an observer can view with both eyes a magnified image of an object which is usually the face plate of an image intensifier tube. The image intensifier tube receives light from a scene at a low brightness level and, by an amplifying electro-optic system, displays on its face plate an image of the scene of sufficient brightness for viewing by a human observer. Light from the displayed image travels via some form of magnifier to the observer's two eyes. In some applications of biocular viewing apparatus the object can be, instead of an image intensifier tube, a cathode ray tube.
In practice, biocular viewing apparatus does not normally present the whole field of view to both eyes and commonly the observer's left eye sees all of the righthand field and only a portion of the left while the right eye sees all of the lefthand field and only a portion of the right. However, this has been found satisfactory and preferably to a monocular arrangement by which the observer sees the image with one eye only.
Biocular apparatus has the advantage of needing only one image intensifier or cathode ray tube over binocular apparatus in which two separate tubes and associated optics are provided, one for each eye. However, in biocular apparatus the optics has to meet the requirement of providing satisfactory imagery for the two spaced eye positions from a single object. This can be achieved by the use of a biocular magnifying lens, i.e. a lens specially designed to have an exit pupil large enough to accommodate both eyes and to be adequately corrected for the two spaced eye positions having regard to the angular difference in geometry over the biocular portion of the field of view. Such a lens is disclosed in British Pat. No. 1,389,564. This can also be achieved by the use of a collimating magnifier lens and a prism arrangement which directs the light output from the lens to the two spaced eye positions. Such a system is disclosed in British Pat. No. 1,506,614.
It is sometimes highly desirable that the biocular viewing apparatus should be mounted actually on the observer's head in the manner of goggles. This has the advantage relative to a fixed mounting of permitting head movement while maintaining a view through the apparatus and relative to hand holding of leaving the observer's hands free to perform other functions. Necessary requirements for head mounting are small size and low weight. These same requirements are, however, also desirable for modes other than head mounting, especially for man portable equipments.
The advent of relatively small and lightweight image intensifier and cathode ray tubes has greatly assisted in reducing the size and weight of the equipment and reasonable portability and even head mounting has been achieved. Nevertheless, there is a continuing demand for reduction in size and weight allied with a desire for low cost while, of course, maintaining satisfactory optical performance.